1. Field of the Invention
The present invention relates to a high-strength steel pipe rockbolt, which is firmly fixed to a bedrock or ground in a state radially expanded by a hydraulic pressure, and a method of manufacturing thereof.
2. Description of Related Art
A steel pipe rockbolt, which is firmly fixed to a bedrock or ground in an expanded state, is manufactured from a hollow shaped pipe having one or more expansive concavities extending in an axial direction. The steel pipe rockbolt 1 has a sealed end, which is inserted into a rockbolt-setting hole formed in a bedrock or ground 2, as shown in FIG. 1. There is a vacancy between the rockbolt-setting hole and the un-expanded steel pipe rockbolt 1 (FIG. 2A). The steel pipe rockbolt 1 is expanded by a hydraulic pressure (FIG. 2B) provided by the hydraulic pump 3, and finally pressed onto an inner wall of the rockbolt-setting hole (FIG. 2C). Consequently, the bedrock or ground 2 is reinforced with the rockbolt 1.
A shaped pipe with at least an expansive concavity 4, which extends along an axial direction, has been used as an expansive rockbolt, in order to facilitate expansion by a hydraulic pressure. The shaped pipe has hermetically sealed top and rear ends and a hole for introduction of a pressurized fluid at its side wall. A shaped steel pipe, which has sleeves fixed to its both ends for introduction of a pressurized fluid, is also disclosed in JP 2003-501573 A.
For standardization of labors and saving of labor costs in construction sites such as tunnels, many rockbolt-setting holes of the same size are drilled in a bedrock or ground 2, and steel pipe rockbolts of the same diameter are placed in the rockbolt-setting holes. For instance, a shaped pipe, which is formed from a steel pipe of 54 mm in outer diameter to a profile having an outer diameter of 36 mm and a concavity 4, is placed in a rockbolt-setting hole of 45-50 mm in size and firmly fixed to a bedrock or ground 2 by hydraulic expansion.
The expansive steel pipe rockbolts are classified to a 110 kN group and a 170 kN group by the yield strength necessary for construction conditions, e.g. competence and geomechanics of a bedrock or ground as well as cross-sectional profiles of tunnels. Rockbolts, which belong to the 110 kN group, are manufactured from steel sheets of 2 mm in thickness with a tensile strength of 300 N/mm2 or more and a total elongation of 30% or more. Rockbolts, which belong to the 170 kN group, are manufactured from steel sheets of 3 mm in thickness with a tensile strength of 300 N/mm2 or more and a total elongation of 35% or more. In any case, the steel sheet is formed to a cylindrical pipe of 54 mm in outer diameter and further reformed to a shaped pipe of 36 mm in outer diameter with a concavity 4.
The shaped pipe is manufactured by partially bending a cylindrical pipe with a small bending radius in a sectional plane, as shown in FIG. 2A. On the presumption that shaped pipes have the same outer diameter, a bending radius at a center is smaller as a thickness is increased for a steel sheet, which is formed to a shaped pipe. The shaped pipes are further swaged at its both ends, since sleeves having inner and outer diameters regularized in size are fixed to the end parts of the shaped pipes. A thicker steel sheet is reformed with a smaller bending radius even in the swaging process. That is, a local bending radius becomes smaller as aria thickness increase of a steel sheet for raising strength of a rockbolt.
By the way, many strains are introduced into a steel sheet in a pipe-making process, a pipe-shaping process and a swaging process. Strains are also accumulated during hydraulic expansion of a shaped pipe. When the shaped pipe is further expanded, it is often cracked due to the introduction of additional strains. Cracking causes leakage of a pressurized fluid, insufficient expansion of the shaped pipe and a shortage of the strength necessary for a rockbolt.